DESCRIPTION The goal of the proposed study is to elucidate the most precise means of measuring individual differences in dopamine (DA) functioning, and then to test the relation of DA reactivity to extraversion. Using a pharmacological challenge protocol with hormonal and behavioral indicators as responses to the challenge, several preliminary studies have demonstrated a strong relation between DA and extraversion of positive emotionality (PEM) (Depue, 1995, 1996; Depue et al., 1994). The most powerful means of expressing individual differences in DA trait levels is to assess the slope of DA reactivity across a series of incentive stimulus magnitudes (i.e. DA-agonist challenge doses), providing a stimulus-DA dose-response function. The proposed study using a randomized cross-over design under double blind conditions to establish a dose-response challenge protocol based on a set of 2 methylphenidate (MP) doses (oral.3, .5 mg/kg) and placebo as a function of sex. Indicators of MP effects are growth hormone (GH) and positive affective ratings to incentive-motivating film clips. Slope will be calculated across the MP doses for each subject for each variable (GH and positive affective ratings) to provide dose-response indicators of DA-reactivity to MP challenge. These indicators will then be correlated with extraversion and its subdomains. This study will contribute to understanding of psychiatric problems that may involve a relation between DA and behavior. Recent research suggests that understanding the relationship between DA and behavior could have benefit in understanding, and perhaps in treating, personality disorders, some forms of affective disorders, schizophrenic positive symptoms, and genetic-experience interactive liabilities to substance abuse that may depend on sensitivity of DA receptor types.